FIG. 3 shows a circuit construction of a prior art microwave transmit receive switch.
In FIG. 3, the microwave transmit-receive switch includes semiconductor switching elements 1a and 1b, such as diodes or transistors, a receiving terminal 2 and transmission terminal 3, and a common signal terminal 4 connected with an antenna. Quarter wavelength transmission lines 21 and 31 connect the switching elements 1a and 1b and the branching point 5 where the line branches from the antenna terminal 4 to the transmission terminal 3 and the receiving terminal 2.
The device will operate as follows.
When a signal from the antenna terminal 4 is to be received, the switching element 1b connected with the transmission terminal 3 is closed and the switching element 1a connected with the receiving terminal 2 is opened. In this state, since the switching element 1b is closed, the characteristic impedance of the transmission line viewed from the branching point 5 toward the transmission terminal 3 is very high, and the signal flows into the receiving terminal 2.
On the other hand, when a signal is transmitted from the transmission terminal 3, the switching element 1a connected with the receiving terminal 2 is closed and the switching element 1b connected with the transmission terminal 3 is opened. Then, the characteristic impedance of the transmission line viewed from the branching point 5 toward the receiving terminal 2 is very high, and a transmission signal is sent out to the antenna terminal 4.
Generally, a transmission signal is a high power signal and the power thereof amounts to about 100 times that of the receiving signal. That is, the transmission signal has a power of about 10 to 20 W. Therefore, in handling such high power signal, the switching element 1b is required to have a high breakdown voltage in its opened state in the prior art microwave transmit-receive switch. In realizing high breakdown voltage in a switching semiconductor element, however, there are production technique limitations, and further the element size has inevitably increased. This has further resulted in a problem in microwave transmit-receive switches in a monolithic microwave integrated circuit (hereinafter referred to as "MMIC"). The cost inevitably increases due to the increase in the area of MMIC chip, accompanied by restricted power handling capability. In addition, in such a microwave circuit, even when the switching elements 1a and 1b are closed, it is not possible to make the switching elements completely short-circuited. That is, even though the characteristic impedances of the quarter wavelength phase delay lines 21 and 31 are 50 .OMEGA., the high frequency resistances which arise in the switching elements 1a and 1b are added thereto, and therefore impedance matching between the quarter wavelength phase delay line 21 and 31 and the respective terminals cannot be obtained due to the inductances of the switching elements. Furthermore, in order to improve impedance matching and obtain a desired impedance, matching circuits are required, resulting in complicated circuitry.